[ios, macos] Snapshotter (runtime styling) delegates

Added a protocol and property for MGLMapSnapshotter’s delegate, which it uses for runtime styling. Generalized MGLStyle to be usable in the absence of MGLMapView. Moved MGLOpenGLStyleLayer code around to compile.

platform/ios/src/MGLMapView+Impl.mm

@@ -50,7 +50,7 @@ void MGLMapViewImpl::onDidFailLoadingMap(mbgl::MapLoadError mapError, const std:
             break;
         case mbgl::MapLoadError::StyleLoadError:
             code = MGLErrorCodeLoadStyleFailed;
-            description = NSLocalizedStringWithDefaultValue(@"LOAD_STYLE_FAILED_DESC", nil, nil, @"The map failed to load because the style can't be loaded.", @"User-friendly error description");
+            description = NSLocalizedStringWithDefaultValue(@"LOAD_STYLE_FAILED_DESC", nil, nil, @"The map failed to load because the style can’t be loaded.", @"User-friendly error description");
             break;
         case mbgl::MapLoadError::NotFoundError:
             code = MGLErrorCodeNotFound;

platform/ios/src/MGLMapView.h

@@ -5,6 +5,7 @@
 #import "MGLGeometry.h"
 #import "MGLMapCamera.h"
 #import "MGLTypes.h"
+#import "MGLStyle.h"
 
 NS_ASSUME_NONNULL_BEGIN
 
@@ -14,7 +15,6 @@ NS_ASSUME_NONNULL_BEGIN
 @class MGLPolyline;
 @class MGLPolygon;
 @class MGLShape;
-@class MGLStyle;
 
 @protocol MGLMapViewDelegate;
 @protocol MGLAnnotation;
@@ -188,7 +188,7 @@ FOUNDATION_EXTERN MGL_EXPORT MGLExceptionName const MGLUserLocationAnnotationTyp
  Simple map view</a> example to learn how to initialize a basic `MGLMapView`.
  */
 MGL_EXPORT
-@interface MGLMapView : UIView
+@interface MGLMapView : UIView <MGLStylable>
 
 #pragma mark Creating Instances
 

platform/ios/src/MGLMapView.mm

@@ -6459,7 +6459,7 @@ public:
         return;
     }
 
-    self.style = [[MGLStyle alloc] initWithRawStyle:&self.mbglMap.getStyle() mapView:self];
+    self.style = [[MGLStyle alloc] initWithRawStyle:&self.mbglMap.getStyle() stylable:self];
     if ([self.delegate respondsToSelector:@selector(mapView:didFinishLoadingStyle:)])
     {
         [self.delegate mapView:self didFinishLoadingStyle:self.style];